1. Field of the Invention
The present invention relates generally to a semiconductor switching circuit using a power MOS transistor as a switching element which switches on and off a current flow through a load, particularly to a MOS (Metal Oxide Semiconductor) device switching circuit having a power MOS transistor protection circuit which interrupts an excessively high drain current flowing through the power MOS transistor by forcefully turning off the power MOS transistor when a switching signal for turning on the power MOS transistor is inputted to the gate of the power MOS transistor with the load short-circuited.
2. Description of the Prior Art
Recently, there is an industrial demand for simplifying and integrating a semiconductor switching circuit which provides on and off control of a current flowing through a load according to a switching signal and for decreasing a power supply voltage applied to the load. Therefore, a power MOS transistor device, particularly a vertical power MOS transistor device having a low on resistance and exhibiting a superior high power switching action has been applied to a switching element of the semiconductor switching circuit.
However, there is a problem that the power MOS transistor will eventually be thermally destroyed due to an excessively large power dissipation between a drain and source of the power MOS transistor, the power dissipation being determined by the product of a drain potential and drain current, when a gate potential of the power MOS transistor is turned to a high level to turn on the power MOS transistor with the load short-circuited, i.e., the power supply voltage is directly applied between the drain and source of the power MOS transistor.
To cope with such a problem, a protection circuit has been proposed, wherein a resistor is intervened between the source of the power MOS transistor and ground so that the change in the drain current thereof is converted to a corresponding voltage. The detected voltage is compared with a predetermined reference voltage by means of a comparator. The compared output signal is sent to a drive circuit intervened between an input terminal through which a switching signal is inputted and the gate of the power MOS transistor so that the drive circuit is actuated to forcefully turn the gate potential of the power MOS transistor to a low level, thus the power MOS transistor turning off. Consequently, the power MOS transistor can be protected from the above-described destruction.
However, there arise many problems in such a conventional switching circuit having the above-described protection circuit.
That is to say, since the resistor described above always consumes wasteful power whenever the power MOS transistor turns on, operation of the switching circuit results in large power consumption. Also since such a low-resistance resistor requires a large area in a semiconductor chip, when the switching circuit is integrated on a semiconductive substrate, it is difficult to integrate the whole switching circuit with high circuit density.
Furthermore, since the substrate itself acts as the drain in a case of a vertical MOS transistor when the vertical MOS transistor having a small on resistance is used as the power MOS transistor and the whole switching circuit is integrated on the same semiconductive substrate, the potential of the substrate is not stable so that it becomes difficult to form other circuit components (e.g., the comparator and drive circuit described above) on the substrate for the power MOS transistor. Consequently, discrete elements of these circuit components are needed to attach externally to the substrate for the power MOS transistor.
In addition, such a conventional switching circuit cannot accurately detect the short-circuiting of the load, when the power supply voltage is varied over a wide range and a large capacity load such as head lamps in an automotive vehicle is used as the load.